The present invention relates generally to a heat exchanger device, and more particularly to a novel compact heat exchanger design configuration using polymeric tubing. The predominant current application for the inventive heat exchanger apparatus is for the temperature control of high purity and/or corrosive fluids.
Many industries require the use of heat exchangers to regulate the temperature of high purity and/or corrosive fluids. For example, microchip fabrication within the semiconductor industry requires heating and temperature regulation of the etching and/or cleaning fluids used to etch and/or clean silicon wafers and microcircuit lines. Because both the process temperatures and the heat capacities of the etching/cleaning fluids are relatively high, a rather large amount of heat is required to raise and maintain the temperature of the etching/cleaning fluid.
Due to the corrosive nature of the typical etching/cleaning fluids used in the semiconductor industry, common materials traditionally utilized in the fabrication of heat exchangers such as metals are not chemically compatible, and therefore, are unacceptable. While metals are extremely good thermal conductors, they are chemically attacked when exposed to these corrosive fluids. As a result, the fluid becomes contaminated and can no longer be used as an etching/cleaning agent.
In order to solve this limitation, a chemically inert material such as Teflon(trademark) is used to either carry the fluid or to protect the resistive element from being corroded, as in the case of an immersion type heater. Although chemically inert, Teflon (trademark) is a very poor conductor, and therefore, the thermal transfer between the heat source and the fluid is limited. There are currently two configurations of heat exchangers that utilize Teflon(trademark) to maintain both chemical compatibility and purity. The first, and most common configuration, is referred to as an immersion heater. The immersion type heaters utilize large vessels with immersed heating coils that are encased by a chemically inert material such as Teflon(trademark). Because Teflon(trademark) is a relatively poor conductor, a very thin layer of Teflon(trademark) is used in order to minimize the thermal resistance between the heating element and the fluid being heated. Also, in order to increase the thermal transfer to the fluid, it is necessary to maximize the surface area between the heating element and the fluid. Therefore, large lengths of the heating element are packed in a coil arrangement inside the vessel. These coils result in xe2x80x9cdeadxe2x80x9d zones where particles reside and shed over time. This makes the described arrangement less desirable for high purity applications. This is unacceptable because, due to stringent process requirements, etching/cleaning fluids must be free of foreign particles in order to avoid the contamination and destruction of microcircuits formed in the silicon wafers.
Another problem associated with immersion type heaters is related to the geometry of such coils. As the fluid flows across these coils, stagnant regions are formed. These are regions where no fluid flow is present and/or regions where no fluid ever comes in contact with the heating element xe2x80x9cmicro bubblesxe2x80x9d. Stagnant regions can lead to xe2x80x9chot spotsxe2x80x9d which are areas where high temperature gradients exist. High temperature gradients can often times degrade the chemical (e.g., lead to premature chemical aging). The combination of hot spots and the micro-bubbles greatly reduce both the efficiency of the heat exchanger and the heating element life, and can also lead to chemical degradation. Another common problem associated with immersion type heat exchangers is that the thin layer of Teflon(trademark) burns immediately when the heating elements become exposed to air. This is a common mode of failure that significantly increases maintenance and parts replacement costs.
Another configuration of heat exchanger currently in use is illustrated in the example of U.S. Pat. No. 5,899,077 issued to Wright, et al., which describes a type of heat exchanger where inert tubing, such as Teflon(trademark) tubing, is sandwiched between thermally conductive rectangular plates. This heat exchanger has been designed to control the temperature of fluids within the room-temperature range. In this configuration, the temperature of the thermally conductive material is controlled via thermoelectric modules. Although thermoelectric modules are useful devices that can cool and heat the conductive material, they are limited to low wattage applications. Hence, this heat exchanger device would not be suitable for heating the common semiconductor etching and cleaning fluids.
Another problem associated with the design described in the prior art listed above, is that it is very difficult to form tight bends in known inert tubing materials. This creates several problems when designing and manufacturing heat exchangers, wherein tubing typically includes multiple bends. First, known inert tubing is easily kinked, and cannot therefore be bent into small diameter bends. Rather, such tubing requires a large bend radius and is, therefore, often bent outside of the heat exchanger, thereby reducing the heating efficiency of the heat exchanger and increasing its size. Further, as the wall thickness of the tubing decreases, the required bend radius increases. Alternatively, if the tubing is entirely retained within the heat exchanger, a complex curved channel with large bend radii must be machined into the conductive plates. In either situation, because of the large bend radii of the plastic tubing, less tubing can be used per unit surface area of the heat exchanger, thereby reducing the thermal efficiency of the heat exchanger and dramatically increasing its size.
In order to compensate for the limited surface area caused by the limited number of bends and limited overall quantity of tubing that can be sandwiched between the rectangular conductive plates, coiled inserts are sometimes placed within the tube. While the turbulence caused by the inserts facilitates increased thermal transfer between the heat exchanger and the fluid, the inserts also cause dead zones within the fluid flow, increasing the potential for particle build-up and contamination of the etching/cleaning fluid. In addition to the coiled inserts, thinner walled inert tubing is often used in order to increase the thermal conduction between the plates and the fluid. While reducing the tubing wall thickness enhances the heat transfer between the conductive plate and fluid, it dramatically reduces the pressure rating of the inert tubing and dramatically increases its bend radius. This severely limits the temperature and pressure ranges within which the heat exchanger can operate, making such solutions unsuitable for many heating applications.
What is needed, therefore, is a heat exchanger design that allows for increased inert tubing surface area while remaining compact. It would also be useful to have a heat exchanger design where no stagnant areas and dead zones exist and/or a heat exchanger that can withstand high pressures at elevated temperatures.
Accordingly, it is an object of the present invention to provide a heat exchanger which can be used with corrosive fluids.
It is another object of the present invention to provide a heat exchanger which is appropriate for use with fluids wherein purity and cleanliness are essential.
It is yet another object of the present invention to provide a heat exchanger which is compact and efficient.
It is still another object of the present invention to provide a heat exchanger which is rugged and reliable in operation.
It is yet another object of the present invention to provide a heat exchanger which is easy and inexpensive to manufacture.
The present invention allows for the heating of high-purity and/or corrosive fluids by utilizing a cylindrical shaped conductive material with integral spiral shaped channels wherein inert tubing is wrapped. The unit is compact, highly expandable, and inexpensive to produce. This invention can also be used for both heating and cooling and is not limited to high-purity and/or chemically aggressive fluids.
The examples of the particular embodiments of the heat exchanger described include at least one cylindrical shaped thermal reservoir and a tube in thermal contact with the thermal reservoir. The tube is formed from a chemically inert material which is perfluoroalkoxy (xe2x80x9cPFAxe2x80x9d) plastic in this particular example, which has relatively high working temperatures (exceeding 250 degrees Celsius). In the disclosed embodiments, the conductive material is cylindrical in shape with integral spiral shaped grooves.
The spiral channels are arranged such that the pitch (the spacing between channels) is slightly greater than the diameter of the tubing. The spiral shaped channel depth is slightly less than the tubing diameter. According to the present invention, the cylindrical thermal reservoir diameter can be made smaller than the natural bend radius of the tubing.
The thermal reservoir(s) of the various heat exchangers can be heated and/or cooled in a variety of ways. In a particular embodiment, at least one heater is inserted into a machined hole in the thermal reservoir(s). In a more particular embodiment, the heater is a cartridge heater disposed in the thermal reservoirs of the heat exchanger. In an alternate embodiment, thermoelectric chips are coupled to the outside of the thermal reservoir. Optionally, a heat sink can be secured to the thermal reservoir to prevent the thermoelectric chips from overheating, as well as, to regulate the temperature within the thermal reservoir.
In an alternate embodiment, an outer cylindrical shaped thermal reservoir, also containing spiral shaped grooves, is coupled to the outside of the primary thermal reservoir. Standard metallic tubing such as copper or aluminum is placed inside the outer cylinder spiral grooves and is used to carry cold fluids such as refrigerant from a condensing unit or chilled water. The cold fluid is used to thermally regulate the temperature within the primary thermal reservoir.
In yet another alternate embodiment, the cold fluid flows directly though the primary thermal reservoir. This construction eliminates the need for an outer thermal reservoir including the copper tubing.
The fluid conduction tubes of the heat exchange sub-units can be configured in a variety of arrangements. For example, the tubes of adjacent heat exchange sub-units can be connected in series or in parallel. Indeed, the heat exchange sub-units of an expanded heat exchanger can be configured in any combination of series or in parallel groups.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of modes of carrying out the invention, and the industrial applicability thereof, as described herein and as illustrated in the several figures of the drawing. The objects and advantages listed or discussed herein are not an exhaustive list of all possible objects or advantages of the invention. Moreover, it will be possible to practice the invention even where one or more of the intended objects and/or advantages might be absent or not required in the application.
Further, those skilled in the art will recognize that various embodiments of the present invention may achieve one or more, but not necessarily all, of the above described objects and/or advantages. Accordingly, the listed objects and advantages are not essential elements of the present invention, and should not be construed as limitations.